The invention concerns a method for steering a vehicle, which is steered via a steering unit by a first steering arrangement, particularly a wheel steering, and a second steering, particularly an articulated steering, and a steering deflection means for detecting the second steering arrangements, with the steering unit operatively connected to both steering arrangements.
A method and a steering system is shown in U.S. Pat. No. 4,565,257. The vehicle described therein can be steered in two ways: firstly by means of a wheel steering, which turns steered wheels in relation to a chassis. Secondly, the vehicle can also be steered by means of an articulated steering. This steering turns two vehicle parts, which are connected via a converting kit, in relation to each other.
Another vehicle, which is equipped with wheel and articulated steering, is shown in U.S. Pat. No. 4,802,545. Also DE 35 43 054 A1 shows a vehicle of this kind. With this vehicle, a steering occurs in that firstly the wheel steering is taken to the end stop, before a working cylinder for the articulated steering is activated.
Normally, such vehicles are used as building site vehicles or as self-propelled working machines. When such a vehicle drives straight ahead, it is substantially unavoidable that in connection with the second steering, particularly with an articulated steering, a deviation of the vehicle from the straight position occurs. The driver may find this deviation unpleasant, as the steering properties of the first steering are influenced by it.
This task is solved in that a dead band is monitored by means of the steering deflection of the second steering and that a correction of the second steering is effected, when the dead band is left and the first steering is activated. This embodiment has several advantages. Firstly, when the second steering exceeds a permissible tolerance area, called a dead band, and only when the dead band is exceeded, is the intervention possible at all. All errors, which are within the range of the dead band, are tolerated. Usually they only have such small influence on the driving behaviour that they can be accepted. However, an intervention, which leads to a correction, does only occur when the driver activates the first steering. Thus, the driver is only very little influenced by the correction. As long as the driver drives straight ahead, no changes occur in the two steerings of the vehicle for the driver does not have to anticipate that a correction will change the driving behaviour of the vehicle in an unforeseen way.
The correction is only initiated, when, during the correction, the second steering steers the vehicle in the same direction as the first steering. This involves the advantage that the comfort for the driver is additionally improved. The driver usually feels nothing when the first steering is overruled by a correcting movement of the second steering.
The correction is initiated by switching the steering unit from the first to the second steering. This involves the advantage that no additional pressure source is required to correct the second steering. During steering, the steering unit supplies hydraulic fluid under pressure, which can then be used for the correction of the second steering.
Further, the correction is initiated at the beginning of a steering procedure. Thus, the actions of both steerings are decoupled. This has no negative influence on the steering behaviour as a whole, as usually the driver steers the vehicle with a feedback, that is, the driver tests visually or by feeling, if the vehicle has assumed the desired direction or not. If this is not the case, he intervenes in the first steering again.
Also, the correction is made in dependence of a rotational speed of a steering hand wheel. Thus, corrections can, for example, be limited to slow steering movements, so that a fast direction change of the vehicle will not be followed by a correcting movement of the second steering. For example, the correction can be limited to steering hand wheel speeds of less than 50 rotations per minute.
The task is also solved by means of a steering system as mentioned in the introduction in that the sensor arrangement has a dead band sensor at the second steering, which sensor is connected with a correcting device, which supplies pressurised fluid to the second steering on leaving a predetermined dead band and on activation of the steering unit.
Thus, the correction of the second steering is limited to such instants, during which the first steering is activated. The dead band sensor, together with the correcting device, acts as a locking element, which does not permit a correction when the first steering has not been activated.
A direction sensor is provided, which monitors a steering direction. The correcting device releases pressurised fluid only when a correcting direction and the steering direction correspond to each other. Thus, the driver is not unpleasantly affected if he steers in one direction, and the vehicle first makes a corrective movement in the other direction.
The correcting device is connected with a steering speed sensor and only releases pressurised fluid when the steering speed is lower than a predetermined value. Thus, corrections are limited to uncritical situations, in which the driver only intends to cause slow steering deflections or directional changes. When, however, the system senses that the driver intends to obtain fast directional changes with the vehicle, corrections do not occur. The information required for this is sent to the system from the steering speed sensor.
A selecting device is arranged between the steering unit and the steerings, and to initiate corrections, the correcting device activates the selecting device. The selecting device can function whether the steering unit must act upon the first steering, or upon the second steering. Apart from this function, the selecting device is able to switch over for a short while, without noticeable influencing the driver to supply the second steering with the amount of oil required to make a correction.
The dead band sensor is mechanically coupled with a control valve. This is a purely mechanical locking, which ensures that a correction can only take place when the second steering leaves its dead band. Only in this case does the control valve open to permit the pressure increase required to make a correction.
Therefore, a principal object of this invention is to provide a method and system for steering a vehicle which improves the driving behaviour of the vehicle.